Seeing heartbeat vs. hearing heartbeat on a vaginal ultrasound

I had my first ultrasound on Tuesday and I am measuring 8wks 2 days. With my first son I remember being able to not only see the heartbeat but also hear it. This time I am at a different doctor's office and I was only able to see the heartbeat and he told me it was 150bpm. Is it normal not to be able to hear it this early through a vaginal ultrasound? Or maybe his sound didn't work or something. He told me everything looked great and he would see me next month. I was just a little curious as to why I couldn't hear it.

Hearing the heartbeat is defiantly more of a precious moment ! I was able to hear my 1st at 8 weeks and put me in tears ... I moved to a different state since and started with a new doc can't get my first appointment until aug 13th ill be 10 weeks then I better be able to hear it by then lol